Methods for cleaning microelectronic substrates using ultradilute cleaning liquids

ABSTRACT

A method of cleaning a surface of an article using cleaning liquids in combination with acoustic energy. Preferably, an ultradilute concentration of a cleaning enhancement substance, such as ammonia gas, is dissolved in a liquid solvent, such as filtered deionized water, to form a cleaning liquid. The cleaning liquid is caused to contact the surface to be cleaned. Acoustic energy is applied to the liquid during such contact. Optionally, the surface to be cleaned can be oxidized, e.g., by ozonated water, prior to cleaning.

[0001] This application is a continuation-in-part application of U.S.patent application Ser. No. 09/311,800 filed May 13, 1999 in the namesof Puri et al. , which application is incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to methods of cleaning the surfacesof articles, particularly microelectronic devices at one or more pointsduring the manufacture of such devices. More particularly, the presentinvention relates to methods of cleaning surfaces of microelectronicdevices using wet processing techniques in conjunction with theapplication of acoustic energy and/or in conjunction with an oxidizingpre-treatment. In preferred embodiments, cleaning liquids of the presentinvention are ultradilute solutions formed by dissolving a gas solute ina suitable solvent.

BACKGROUND OF THE INVENTION

[0003] Since the early days of the microelectronic industry, theimportance of minimizing contamination on microelectronic devicesthroughout the manufacturing process has been recognized. Contaminantsinclude particles, photoresist residue, and/or the like whose presencecan adversely impact the performance and function of microelectronicdevices if not adequately removed. Accordingly, various cleaningtreatments have been devised.

[0004] However, as the end product devices have become more and moreminiaturized, a contaminant occupies an increased percentage of theavailable space for circuitry and other device elements. Hence,cleanliness of the materials has become far more critical, andcleanliness specifications have become increasingly more stringent.Unfortunately, these trends make cleaning much more challenging.

[0005] One traditional cleaning approach has involved the use of anaqueous solution of H₂O₂ and NH₄OH to carry out cleaning in which thevolume ratio of the peroxide to aqueous ammonia to water typically is1:1:5. These relatively concentrated mixtures are commonly referred toas the Standard Cleaning Solution 1 (“SC-1”). The SC-1 process isbelieved to detach particles through surface etching. The use ofmegasonic energy in combination with the SC-1 chemistry is reported toimprove particle detachment.

[0006] In basic solution, many common particle types will bear the samekind of surface charge as do silicon and silicon dioxide surfaces beingcleaned, theoretically causing the particles and the surfaces to repeleach other via electrostatic repulsion. However, SC-1 cleaning solutionstend to have very high ionic strength, causing the electrostaticrepulsion forces to be negligible as a practical matter. Electrostaticrepulsion forces therefore play only a minor role, if any, in connectionwith the SC-1 chemistry.

[0007] Although widely used, the SC-1 approach has drawbacks. Theserelatively concentrated solutions might clean effectively, butunfortunately they can also deposit metal contaminants onto the devicesbeing cleaned. Of course, a cleaning method that deposits contaminantsis counterproductive. These concentrated solutions can also unduly etchand damage the surfaces of the devices being cleaned. Device damage isalso a result that is desirably avoided by a cleaning method. As stillanother drawback, the peroxide typically must incorporate stabilizers,and these can contaminate the surface being cleaned, which also iscounterproductive especially when relatively high concentrations ofperoxide are used.

[0008] Another cleaning approach is described in U.S. Pat. No.5,656,097. This approach involves cleaning devices with aqueoussolutions of ammonia and hydrogen peroxide in combination with theapplication of megasonic energy. In this approach, the dilute solutionsare prepared by diluting more concentrated solutions of aqueous ammoniawith water. The approach has drawbacks. Although dilute, these solutionscan still unduly etch the devices being cleaned, metal contaminants canstill be deposited, and the stabilizer for the peroxide is still acontaminant. Further, it is very difficult to prepare dilute solutionswith good accuracy by diluting relatively small volumes of concentratedsolutions with relatively large volumes of solvent. The inaccuracy canlead to differences in cleaning performance from device to device.

[0009] It can be seen, therefore, that improved cleaning methods thatcan satisfy these more stringent demands imposed by miniaturization arestill needed.

SUMMARY OF THE INVENTION

[0010] The present invention provides an approach for cleaning articles,such as microelectronic devices at various stages of manufacture, thatis extremely effective at removing particle contaminants and/or organicdebris, e.g., photoresist remnants, from the device surfaces. Inpreferred embodiments, the approach accomplishes cleaning withoutdepositing metal contaminants onto the surface of the devices andwithout undue etching or other damage of the surfaces. As used herein,the term “microelectronic device” includes but is not limited tosemiconductor wafers, integrated circuits, thin film heads, flat paneldisplays, microelectronic masks, and the like. The term shall also referto partially completed devices as they are being manufactured.

[0011] The approach of the present invention is significant in at leasttwo respects. First, preferred embodiments of the present inventionachieve cleaning efficiencies of better than 99.9% with respect toparticles having a size greater than about 0.16 microns. Second, inaddition to high particle removal efficiency, it is also important tocarry out cleaning operations without adversely affecting surfacesmoothness and without depositing additional contaminants, e.g., metalcontaminants, onto the surface being cleaned. Preferred embodiments ofthe present invention do not adversely affect surface roughness in anysignificant way. In fact, preferred cleaning embodiments of the presentinvention have actually provided post-clean surfaces that are smootherthan the same pre-cleaned surfaces Metal contamination of preferredembodiments is neutral, meaning that cleaning operations depositsubstantially no, if any, metal contaminants onto the surfaces beingtreated.

[0012] Preferred embodiments of the invention carry out cleaningoperations using the combination of acoustic energy and a cleaningliquid comprising an ultradilute concentration of a cleaning enhancementagent. Amazingly, the combined use of acoustic energy, particularlymegasonic energy, and ultradilute cleaning reagents provides exceptionalcleaning performance even though the amount of cleaning enhancementagent in the reagent is almost negligible as a practical matter. Forexample, reagents containing approximately 100 ppm gaseous anhydrousammonia dissolved in filtered deionized water remove particles fromsubstrates such as semiconductor wafers with very high efficiency.

[0013] In one aspect, the present invention provides a method ofcleaning a surface of an article using ultradilute cleaning liquids incombination with acoustic energy. An ultradilute concentration of acleaning enhancement substance, such as ammonia gas, is dissolved in aliquid solvent, such as filtered deionized water, to form a cleaningliquid. The cleaning liquid optionally may also include otheringredients, such as hydrogen peroxide, if desired, but such additivesare not needed and may not even be desired to achieve excellent cleaningperformance. The cleaning liquid is caused to contact the surface to becleaned. Contact can occur by causing the liquid to flow past thesurface, by spraying the liquid onto the surface, by submerging thesurface in a body of the liquid, and/or the like. Preferably, acousticenergy is applied to the liquid during such contact.

[0014] In another aspect, the present invention provides a cleaningmethod in which a surface of an item to be cleaned is first contactedwith a processing liquid comprising an oxidizing agent. A preferredprocessing liquid for this purpose is ozonated water, but solvents suchas water containing other oxidants such as hydrogen peroxide could alsobe used if desired. Next, the surface is contacted with a cleaningliquid, preferably ultradilute aqueous ammonia. Acoustic energy isdirected into the cleaning liquid during at least a portion of the timethat contact with the cleaning liquid is occurring.

[0015] In another aspect, the present invention provides a cleaningmethod in which a substrate is positioned in a cleaning vessel with thesurface to be cleaned being substantially vertical. A cleaning liquidcomprising and ultradilute concentration of a cleaning enhancementsubstance, preferably ammonia, is then introduced into the vessel. Asthe vessel fills, the rising top surface of the cleaning liquidtraverses the substrate surface. Acoustic energy is applied to therising cleaning liquid.

[0016] In still another aspect, the present invention involves a methodof cleaning a surface of an article in which an ultradiluteconcentration of a gaseous cleaning enhancement substance is dissolvedin a liquid solvent to form a cleaning liquid. The cleaning liquid iscaused to contact the substrate surface. While causing the cleaningliquid to contact the substrate surface, acoustic energy is applied tothe cleaning liquid. After causing the cleaning liquid to contact thesubstrate surface, the substrate surface is rinsed and then dried.Preferably, drying occurs by contacting the substrate surface with afirst process reagent comprising a carrier gas, preferably nitrogen, anda cleaning enhancement substance, preferably an ultradiluteconcentration of isopropyl alcohol. The substrate surface is alsocontacted with a drying reagent comprising a heated gas, preferablyheated nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic flow chart of a preferred system forpracticing the principles of the present invention.

[0018]FIG. 2 is a schematic diagram of a single tank system of thepresent invention suitable for pretreating devices with ozonated waterand then following up by cleaning the devices with ultradilute, aqueousammonia.

[0019]FIG. 3 is a schematic diagram of a twin-tank system of the presentinvention in which the system includes cleaning capabilities inaccordance with the present invention as well as rinse/dry capabilities.

DETAILED DESCRIPTION

[0020] For purposes of illustration, the principles and methods of thepresent invention will now be described in connection with system 10shown in FIG. 1. One or more substrates, such as semiconductor wafer 12,are positioned inside cleaning vessel 14. In a preferred orientationwhen processing liquids are introduced into cleaning vessel 14 from thebottom (as is shown), wafer 12 is positioned so that the surface 16 ofwafer 12 to be cleaned is oriented substantially vertically duringcleaning operations. Substantially vertically means that the surface istilted at an angle in the range from 0 degrees to about 10 degrees. Forplanar substrates such as semiconductor wafer 12, a slight tilt awayfrom vertical is desired in order to help prevent adjacent substratesfrom being jostled against each other, or against any carrier (notshown) in which the wafers are held, as processing fluid flows past thesubstrates. Such tilting is optional and may not be desirable in someprocesses. For example, 200 mm wafers need not be tilted at an angle dueto their mass, but 150 mm wafers can advantageously be tilted at anangle of 7 degrees to 8 degrees. This principle of supporting planarsubstrates at a slight tilt away from vertical is described further inU.S. Pat. No. 5,571,337.

[0021] A flow of cleaning liquid from cleaning liquid source 18 isintroduced into cleaning vessel 14 through inlet end 20 and isdischarged through outlet end 21. Cleaning vessel 14 preferably is madeof material that is as transmissive as practically possible to acousticenergy so that acoustic energy from acoustic energy source 22 (describedfurther below) is applied to the contents of cleaning vessel 14 withminimal energy loss due to reflection or absorption of the energy by thewalls of cleaning vessel 14. The material used to form cleaning vessel14 should also shatter resistant and strong enough so that the materialresists cracking, chipping, or shattering during use. Examples of suchmaterials include PTFE available under the trade designations Teflon orHylar), quartz, combinations of these, and the like. Of these, quartz ispreferred. When processing liquids are introduced into cleaning vessel14 through inlet structure (not shown in FIG. 1) such as a sparger,manifold, other flow distribution device, or the like, such inletstructure, to the extent that it is in the path of acoustic energydirected into cleaning vessel 14 also is made of an acoustic energycompatible material such as quartz.

[0022] Cleaning vessel 14 can be any kind of cleaning vessel that allowsa flow of cleaning fluid to be maintained past wafer 12. For example,cleaning vessel 14 can be the cascade, overflow vessel incorporated intothe Series 6000 equipment commercially available from Yield UpInternational, Inc., Mountain View, Calif.

[0023] As the cleaning liquid flows through cleaning vessel 14, theliquid contacts and helps to clean, e.g., remove particles and/or otherimpurities, from surface 16 of wafer 12. The cleaning liquids of thepresent invention generally include a solvent and cleaningly effectiveamounts of one or more cleaning enhancement substances. Examples ofsolvents that can be used in the practice of the present inventioninclude deionized water as well as isopropyl alcohol, ethanol, methanol,other polar organic solvents. Of these, deionized water is mostpreferred. Examples of cleaning enhancement substances include NH₃,NH₄OH, HCl, HF, ozone, and the like. Generally, acid cleaningenhancement substances are suitable for cleaning metal surfaces, whereasbasic cleaning enhancement substances such as ammonia are suitable forcleaning nonmetallic surfaces such as silicon and silicon oxidesurfaces. For cleaning liquids comprising ultradilute concentrations ofa cleaning enhancement substance in a solvent (described further below),it is preferred that the cleaning enhancement substance be a gaseoussolute dissolved in the solvent. When the solute is supplied as gas tobe dissolved into the solvent, the cleaning liquid can be formed bymixing appropriate flows of the solvent and the gas. This allows theultradilute concentration of the solute in the solvent to be controlledwith great accuracy. Gases can also be filtered to a degree not possiblewith liquids, meaning that the gas solute can be provided with a highlevel of purity.

[0024] In the practice of the present invention, any deionized waterincorporated into any processing liquid that is caused to contact wafer12 preferably is electrostatically filtered using the Clean Point®filtration unit commercially available from Yield Up International,Mountain View, Calif. This particular filtration unit uses a series ofpositive and negatively charged filters and is extremely effective atremoving extremely particles from process water with only a minimalpressure drop through the filtration unit. The features and operation ofsuch filters are further described in U.S. Pat. No. 5,571,337.

[0025] The concentration of the cleaning enhancement substance in thecleaning liquid will depend upon how the principles of the presentinvention are being practiced. For instance, in embodiments of thepresent invention in which cleaning operations comprise an oxidizingpretreatment (described below), the concentration of the cleaningenhancement substance in the cleaning liquid can vary within a wideeffective range. Preferably such concentration is ultradilute. The term“ultradilute” preferably means that the volume ratio of the solvent tothe cleaning enhancement agent is in the range from 500:1 to 500,000:1,preferably 1000:1 to 300,000:1, more preferably 100,000:1 to 200,000:1.In some instances in which it may not be practical to determine a volumeratio, then these ratios may be determined on a molar basis.

[0026] Surprisingly, it has now been discovered that ultradilutesolutions of cleaning enhancement substances are extremely effective andbeneficial cleaning liquids for use in the manufacture ofmicroelectronic devices. The fact that ultradilute solutions have anycleaning effect at all is unexpected, yet such cleaning compositionsoffer numerous advantages. First, ultradilute solutions causeinsignificant, if any, etching of the surface being cleaned. Forexample, studies have shown that only a few angstroms, if even thatmuch, of the native oxide surface is etched when cleaning occurs with anultradilute cleaning liquid. Thus, a surface cleaned with an ultradilutecleaning liquid is less affected than if it were to be cleaned with moreconcentrated solutions that can etch tens, if not hundreds, of angstromsof the surface during a cleaning operation.

[0027] Second, ultradilute solutions effectively clean an extremely highpercentage of particles from surfaces of microelectronic devices, whichis a result that is counter to conventional wisdom in the manufacture ofmicroelectronic devices. Conventional wisdom has suggested that theactive cleaning agent in a cleaning liquid must be present at highenough concentrations in order to undergo primarily a bulk reaction,e.g., etching, with the substrate surface. The belief has been that asurface must be measurably etched in order to remove particles from thesurface. However, bulk reactions are not always desirable since suchreactions can damage, e.g., unduly roughen, the substrate in ways thatare difficult to control. Surprisingly, ultradilute cleaning liquidsaccomplish removal of particles from substrate surfaces withsubstantially no etching of the substrate surface. As used herein,substantially no etching means that 10 angstroms or less, morepreferably 5 angstroms or less, of the surface is etched by the cleaningliquid. In some instances, we have even observed no measurable etchingof the substrate surface after cleaning.

[0028] While not wishing to be bound by theory, a possible rationale toexplain the unexpected cleaning capabilities offered by ultradilutecleaning liquids can be suggested. It is believed that the ultradiluteconcentration of a cleaning enhancement substance alters the pH of thecleaning liquid enough so that the zeta potential characteristics ofparticle contaminants on the substrate surface and/or of the substratesurface itself are altered when the surface is being cleaned with a flowof these solutions. Specifically, it is believed that the zeta potentialalteration causes the substrate surface and the particles to becharacterized by opposite surface charges so that electrostaticrepulsion between the particle contaminates and the surface helps toeject the particles away from the surface where the particles are moreeasily caught up and carried away by the flowing cleaning liquid. Beingultradilute, the solutions have relatively low ionic strength, and theelectrostatic repulsion forces can play a key role in particle removal.Thus, ultradilute cleaning liquids are believed to benefit frominterfacial reactions occurring at the liquid-solid interface betweenthe cleaning liquid, on the one hand, and the substrate surface andparticles, on the other hand. This is a completely different cleaningmechanism than the bulk reactions more characteristic of moreconcentrated cleaning liquids.

[0029] Still yet another advantage of using ultradilute solutions isthat cleaning occurs rapidly. For example, five minutes, more preferablythree minutes, is adequate for cleaning the substrate surface 16. Thisshort cleaning duration means fast cycle times. Yet, on the other hand,if it were to be desired for some reason to carry out cleaning for alonger duration, that could be done without damaging substrate surface16 since the cleaning liquid has very little effect upon the substratesurface due to the ultradilute concentration of the cleaning enhancementsubstance.

[0030] As another advantage, it is believed that ultradilute solutionsdeposit insignificant amounts, if any, of metal contaminants onto thesurfaces being cleaned.

[0031] Cleaning liquids may, if desired, be prepared by diluting aconcentrated liquid solution of the cleaning enhancement substance withthe desired solvent. However, when the cleaning liquid contains only anultradilute concentration of the cleaning enhancement substance, it canbe very difficult to accurately control such dilution when the dilutionoccurs continuously over a period of time (as opposed to batchwisedilution) to support a steady state cleaning operation. As aconsequence, unfortunately, the concentration of the cleaningenhancement substance in the diluted cleaning liquid can vary quite abit. This variation in concentration may adversely affect theperformance of the cleaning operations. In contrast to the dilutionapproach, it is substantially easier to establish and maintain a uniformultradilute concentration of the cleaning enhancement substance in thesolvent when the cleaning enhancement substance is supplied as a gas.Appropriate flow rates of the solvent and the gaseous cleaningenhancement substance are easily established and mixed together to forma cleaning liquid with the desired ultradilute concentration. A numberof gas/liquid mixing devices are known for accomplishing this. Apreferred gas/liquid mixer is commercially available under the tradedesignation from Legacy Systems, Inc.

[0032] In addition to cleaning liquid source 18, one or more othersources of processing fluids, such as a gas, liquid, slurry, and/or thelike, may optionally also be fluidly coupled to cleaning vessel 14 sothat such other processing fluids may also be used to treat surface 16of wafer 12 before, during, or after cleaning operations. For purposesof illustration, FIG. 1 shows system 10 as including two additionalsources 24 and 26 of processing fluids.

[0033] In preferred embodiments, optional fluid source 24 is a source ofa rinsing fluid that is present so that surface 16 optionally can berinsed before and/or after being treated with the cleaning liquid and/orany other processing liquid. A preferred rinsing fluid is deionizedwater that has been electrostatically filtered using the Clean Point Rfiltration unit.

[0034] Optional fluid source 26 is preferably an oxidizing processingliquid comprising a suitable solvent and an amount of an oxidizing agenteffective to oxidize the surface of the wafer 12 without undulyaffecting its physical (e.g., surface smoothness characteristics) andfunctional characteristics (e.g., electronic characteristics ofoperational structures formed on or in the wafer 12, if any). Such anoxidizing fluid may be used to oxidize surface 16 of wafer 12 beforetreatment of surface 16 with the cleaning liquid from cleaning liquidsource 18. Advantageously, such an oxidizing pre-treatment enhances theeffectiveness of the cleaning operation for at least three reasons.First, the oxidizing pre-treatment and subsequent treatment with thecleaning liquid work together to more effectively clean substratesurface 16, because the oxidizing pre-treatment it oxidizes organicmaterials on the surface, such as photoresist remnants, which may thenbe easier to remove. The oxidizing treatment thus boosts cleaningperformance because it extends the cleaning effect to a class ofmaterials, organic materials, that otherwise might not be effectivelyremoved by the subsequent treatment with the cleaning liquid. Oxidizingtreatment is thus desirable when surface 16 is known to have organiccontaminants such as photoresist remnants.

[0035] Second, the oxidizing treatment also forms an oxide barrier layeron the surface of substrate 12 that protects the substrate from beingdamaged by the cleaning liquid when such protection may be desired. Suchdamage is more of a concern when the cleaning liquid might be an etchantof the substrate surface being cleaned. For example, silicon can reactwith aqueous ammonia solutions. In contrast, silicon oxide tends to beless reactive with such aqueous ammonia solutions. Accordingly, when thecleaning liquid is aqueous ammonia and the substrate surface 16 includessilicon, it can be desirable to oxidize the silicon surface of thesubstrate to form a protective oxide barrier over the silicon beforecontacting the substrate surface 16 with the aqueous ammonia.

[0036] Third, the resultant oxide surface tends to enhance the cleaningeffect when the surface comprises silicon being oxidized to siliconoxide. In particular, it has been observed that cleaning a siliconsurface with ultradilute aqueous ammonia is more effective inconjunction with an oxidized surface. Although the reason for this isnot known, it is believed that the electrostatic repulsion forcesbetween substrate surface 16 and particle contaminants are stronger whensurface 16 is oxidized.

[0037] Preferred oxidizing compositions are aqueous solutions containingdeionized water as a solvent and an oxidizing agent selected from ozone,hydrogen peroxide, nitric acid, combinations of these, and the like.Preferably, the oxidizing composition is ozonated water containing anultradilute concentration of ozone. More preferably, the oxidizing agentis ozonated water containing 5 ppm to 100 ppm, preferably 10 ppm to 50ppm, most preferably about 17 ppm ozone. Use of ultradilute ozonatedwater is advantageous because such compositions cause the formation of athin, protective oxide layer without otherwise affecting the wafer 12 inany significant way. Ozonated water also is very easy to purify. Incontrast, oxidizing agents such as hydrogen peroxide are much moredifficult to purify and also contain stabilizers which are themselvescontaminants. Oxidants such as nitric acid are less desirable than ozonein that aqueous nitric acid is also a strong etchant and therefore mayalso tend to etch the substrate too much.

[0038] System 10 also includes source 22 of acoustic energyoperationally coupled to cleaning vessel 14 so that acoustic energy canbe directed into cleaning vessel 14. Examples of acoustic energy includesonic, supersonic, ultrasonic, and megasonic energy. The use ofmegasonic energy is preferred in that megasonic energy is most effectiveat removing smaller particles from substrate surface 16. Equipment forgenerating acoustic energy is commercially available from a number ofvendors. Particularly preferred equipment for applying megasonic energyto cleaning vessel 14 includes a generator model no. 68101 and a series98S or series 7857S transducer, or the like, commercially available fromKaijo Corp. Advantageously, use of this particular equipment allowsacoustic source 22 and cleaning vessel 14 to be made of easily separablestructures so that either unit can be removed for service, replacement,or repair as needed. This significantly lowers the cost of maintainingsystem 10 in that one of the cleaning vessel and acoustic source 22 canbe serviced without touching the other. In contrast, many previouslyknown designs integrate the acoustic source and cleaning vessel so thatindependent service, replacement, and repair are not practical. Such aninterdependent design is more expensive to maintain since replacement ofone part would necessitate replacement of both parts.

[0039] In one particularly preferred embodiment of the presentinvention, the cleaning liquid is most preferably an ultradilute,aqueous ammonia solution formed by dissolving anhydrous ammonia gas infiltered, deionized water. It has been found that this cleaning liquidprovides extremely effective cleaning performance, particularly when thesurface to be cleaned comprises silicon oxide. In another particularlypreferred embodiment of the present invention suitable for cleaningsilicon surfaces, the surfaces are first oxidizing by treatment withozonated water and then treated with a cleaning liquid in the form of anultradilute, aqueous ammonia solution formed by dissolving anhydrousammonia gas in filtered, deionized water. It has been found that thiscombination provides an extremely high level of cleaning performance,particularly when the substrate surface to be cleaned is hydrophobicbefore treatment and/or includes organic contaminants.

[0040] In a preferred mode of operation, an optional flow of rinsingliquid from source 24 is established through cleaning vessel 14.Substrates 12 may be positioned in cleaning vessel either before orafter this flow is established. The substrates in the case of wafers,masks, disks, flat panels, liquid crystal displays, thin film heads,photomasks, lenses, and the like, can be face to face, back to back,face to back, or back to face. Face to face and back to back is apreferred orientation. This flow may occur at room temperature for 2 to5 minutes using flow rates appropriate to the type of equipment beingused in accordance with instructions provided by the vendor. Forexample, in a cascade rinse vessel of the type incorporated into theequipment sold by Yield Up International, Inc., the flow rate in thisand all other processing steps of the preferred mode of operation may bein the range from 0.1 to 50, preferably 0.5 to 10, more preferably about5 gallons per minute.

[0041] Optionally, a flow of oxidizing liquid from source 26 may replacethe flow of rinsing liquid through cleaning vessel 14 in order tooxidize substrate surface 16. This flow may be established with orwithout dumping the rinse liquid first. Preferably, however, the flow ofoxidizing liquid is established without dumping the rinse liquid whenthe oxidizing liquid flow is established. The length of time andtemperature for carrying out this treatment will depend upon factorsincluding the composition of the oxidizing liquid, the nature ofsubstrate surface 16, and the like. For ozonated water containing about5 to 100, preferably 10 to 60, more preferably about 17 ppm ozone,treating wafer 12 for 2 to 5 minutes at room temperature would besuitable. These conditions allow an oxide layer to form on substratesurface having a thickness in the range of 8 angstroms to 11 angstroms.Shorter processing times may be used, but the formation of oxide may beincomplete. Longer times may offer no additional benefit, thusincreasing cycle time and expense without good reason.

[0042] Next, wafer 12 is again optionally rinsed with the rinsingliquid, preferably filtered deionized water, in order to wash away theoxidizing liquid. This rinsing step may occur under the same range ofprocessing conditions that are suitable for the first rinsing step notedabove, except that the rinsing liquid preferably is heated for thisrinse. If heated, the temperature of the rinsing liquid preferably maybe any temperature in the range from about ambient temperature to 85° C.The ozonated water of the previous step may be optionally dumped priorto introducing the rinsing liquid.

[0043] Next, a flow of the cleaning liquid through cleaning vessel 14 isestablished. This can be done with or without dumping the rinsing liquidfirst. However, in embodiments in which the cleaning liquid isultradilute with respect to the cleaning enhancement substance, it ispreferred to dump the rinsing liquid first before establishing acascading flow of the cleaning liquid. Dumping may take more time, butcleaning performance is dramatically better. While the reason for thisis not known, a possible rationale can be suggested. When the rinseliquid is dumped, cleaning vessel 14 is empty, at least in the sensethat no part of wafer 12 is submerged in processing liquid when cleaningliquid is introduced into cleaning vessel 14. Accordingly, in acascading rinse cleaning vessel, the top surface of cleaning liquidrises and traverses across surface 16 as cleaning vessel 14 fills. It isbelieved that the zeta potential effects are quite strong at the movinginterface between the liquid surface and the substrate surface 16,facilitating particle removal.

[0044] Treatment with the cleaning liquid may be carried out at anyconvenient temperature within a wide range. For example, the cleaningliquid can be chilled to any temperature below ambient, but above whichthe cleaning liquid freezes, at ambient temperature, or heated to atemperature above ambient but below which the cleaning liquid wouldboil. For example, for aqueous cleaning liquids comprising ultradiluteconcentrations of ammonia (i.e., ultradilute, aqueous ammoniumhydroxide), the cleaning liquid preferably can be supplied at anytemperature or temperatures in the range from 0° C. to 98° C.,preferably 20° C. to about 85° C. The cleaning liquid and cleaningvessel can be pressurized if it is desired to expand the temperaturerange within which the cleaning liquid could be supplied in the liquidphase.

[0045] Excellent cleaning results are obtained by heating the cleaningliquid to elevated temperatures in the range from about 60° C. to about85° C., which are typical temperatures used in connection with the SC-1chemistry. However, unlike SC-1 chemistry, excellent cleaningperformance can be achieved with ultradilute cleaning liquids of thepresent invention at unconventionally low cleaning temperatures. Forexample, the performance of SC-1 chemistry generally is poor at lowtemperatures below about 60° C., particularly below about 30° C. becausethe etching rate of the cleaning liquid slows down too much at suchlower temperatures. In contrast, the cleaning performance of ultradilutecleaning liquids of the present invention is maintained, and is evenimproved in key aspects, as the temperature of the liquid is reduced.Accordingly, preferred embodiments of the invention involve supplyingthe cleaning liquid at a temperature below about 30° C., preferably fromabout 0° C. to about 25° C.

[0046] During the cleaning step, acoustic energy, preferably megasonicenergy, is directed into the cleaning liquid. A suitable acoustic rangeis about 3 watts/cm² or less.

[0047] Following treatment with the cleaning liquid, the cleaned wafer12 can be processed further in any desired way, depending upon whatstage of manufacture the wafer is at. In some instances, the substratemay be subjected to additional manufacturing steps. In other instances,the wafer may be simply rinsed and dried. Preferred rinsing and dryingis accomplished by removing the wafer 12 from the cleaning vessel andtransferring the substrate 12 to a second treatment vessel in which anSTG™ rinse/dry treatment is carried out. The equipment for performingsuch a treatment, is described in U.S. Pat. No. 5,571,337. Additionally,preferred equipment for carrying out an STG™ rinse/dry is commerciallyavailable from Yield Up International, Inc.

[0048] As an overview, the STG™ process involves first rinsing the itemsto be rinsed and dried with a cascading flow of filtered deionizedwater. This rinse occurs in a vessel having a cascade overflow designwith a blanket of low flow, hot nitrogen. The level of deionized wateris then slowly dropped. A typical drop rate is on the order of 0.5 mm/sto 15 mm/s, preferably 1 mm/s to 2 mm/s. As the liquid level is dropped,the low flow, hot nitrogen is maintained, but a flow of nitrogencontaining an ultradilute concentration of a cleaning enhancementsubstance, such as isopropyl alcohol, is also introduced. When the waterlevel drops below the items being rinsed and dried, the liquid contentsof the vessel are quickly dumped, e.g., at a rate of 20 mm/s. This quickdump occurs while the flows of the hot nitrogen and the nitrogen/IPA mixare maintained. After the quick dump of the liquid contents, the flow ofnitrogen/IPA is stopped while the flow of the hot nitrogen is increasedto a higher rate to dry the items, e.g., 300 liters/min to 600liters/min. The resulant dried items can then be removed for furtheruse, processing, storage, or the like. For this process, the nitrogenand nitrogen/IPA are preferably filtered. The hot nitrogen is heated toabout to 130° C. but is believed to be at a temperature of about 50° C.in the rinse/dry vessel when it enters the rinse/dry vessel throughnozzles in the lid of the rinse/dry vessel.

[0049] In order to more concretely illustrate the principles of thepresent invention, FIG. 2 is a schematic illustration of a preferredsystem 100 suitable for cleaning one or more substrates 106 usingcascading flows of deionized water, ozonated water, and/or ultradilute,aqueous ammonia as processing liquids. The principles of system 100 maybe carried out in actual practice using a Model 6200 or other Series6000 apparatus commercially available form Yield Up International,Mountain View, Calif.

[0050] System 100 generally includes cleaning vessel 102 containedinside housing 104. Housing 104 may include one or more access panelssuch as lid 103 in order to allow access to the interior of housing 104for purposes of loading and unloading substrates, maintaining system100, conducting repairs, and the like.

[0051] Cleaning vessel 102 is suitable for establishing a cascade flowof one or more process liquids past one or more substrates 106positioned inside cleaning chamber 108. Cleaning vessel 102 is formedfrom inner tank 110 defining cleaning chamber 108 and outer tank 112. Atleast inner tank 110, and preferably outer tank 112 as well, arepreferably made of a material such as quartz that absorbs as littleacoustic energy as possible. Conventionally, substrates 106 aresupported inside cleaning chamber 108 on a suitable carrier (not shown).In the case of semiconductor wafers, one representative wafer carrier isdescribed in U.S. Pat. No. 5,571,337. Processing liquid enters cleaningchamber through inlet 114 positioned at the bottom of inner tank 110.The outlet of cleaning chamber 108 is formed by rim 116. A cascadingflow of processing liquid introduced into cleaning chamber 108 throughinlet 114 thus flows upward and cascades over rim 116 into outer tank112 from which the processing liquid can be recycled or discarded asdesired through valved drain line 109.

[0052] The cascading flow of processing liquid can be established pastsubstrates 106 either before or after substrates 106 are positionedinside cleaning chamber 108. If substrates 106 are placed into cleaningchamber 108 before the cascading flow is established, then, as is shown,top surface 118 of processing liquid 120 will rise until processingliquid 120 fills inner tank 110 and then cascades over rim 116 intoouter tank 112. As inner tank 110 fills and top surface 118 ofprocessing liquid 120 rises, top surface 118 traverses upward and acrosssurface 107. This kind of traversal is particularly desirable whenprocessing liquid 120 is ultradilute aqueous ammonia in that cleaningperformance is much better when this traversal occurs. As noted above,the reason for this is not known with certainty, but it is believed thatelectrostatic repulsion between surface 107 and particles on surface 107are strongest at the interface between surface 107 and top surface 118of processing liquid 120. Thus, as top surface 118 of processing liquid120 moves up surface 107 of substrate 106, electrostatic repulsion isbelieved to help knock particles off the surface and 120 into the bodyof processing liquid 120 where the particles are more easily carriedaway.

[0053] Processing liquid from chemical supply line 150 is introducedinto inner tank 110 through sparger 122 positioned at the bottom ofinner tank 110. Sparger 122 is a conduit comprising a plurality ofbottom orifices 124 through which processing liquid 120 is dischargedinto inner tank 110. By discharging processing liquid 120 downward intoinner tank 110, currents and eddies against substrates 106 are minimizedand laminar flow through inner tank 110 is more easily achieved. Sparger122 is preferably made of a material such as quartz that absorbs aslittle acoustic energy as possible. Cleaning vessel 102 is positionedover megasonic energy source 126 containing megasonic device 128 andfluid coupling 130 through which megasonic energy is applied to thecontents of inner tank 110. The commercially available megasonic energysystem listed above and commercially available from Kaijo Corp. becausesuch a system allows cleaning vessel 102 to be removably coupled tosystem 100 so that cleaning vessel 102 independently can be easilyremoved for maintenance, repair, or replacement.

[0054] Optionally, although not required, it may be desirable tomaintain a blanket 132 of clean, inert gas over the top of cleaningvessel 102 to minimize the exposure of substrates 106 to particles andother contaminants in the ambient. When such a blanket 132 of inert gasis desired, inert gas such as N₂ or the like can be introduced intovolume 134 above cleaning vessel 102 through gas supply line 136. Theflow rate of inert gas through gas supply line 136 is controlled byvalve 138. Although not shown in FIG. 2, the inert gas preferably isfiltered to ensure that the inert gas supplied to volume 134 is clean.The inert gas can be supplied from any suitable source, but ispreferably supplied from reservoir 140. Reservoir 140 contains a reserveof inert gas that can quickly fill housing 104 when processing liquid isdumped from cleaning vessel 102 through drain line(s) 142. The inert gasan be exhausted from housing 104 through gas exhaust line 144. Flow ofinert gas through gas exhaust line 144 can be controlled by valve 146.

[0055] Valve 148 on chemical supply line 150 can be adjusted to controlthe flow of processing liquid introduced into inner tank 110 throughsparger 122. The bottom of inner tank 110 is also fitted with one ormore drain valves 143 on one or more drain lines 142 so that thecontents of inner tank 110 can be quickly dumped to shorten cycle time.

[0056] One or more processing liquids are supplied to chemical supplyline 150 from chemical supply 152. As shown, chemical supply 152preferably provides the capability of delivering processing liquidscomprising deionized water, ozonated water, and/or ultradilute aqueousammonia to chemical supply line 150. With respect to supplyingultradilute aqueous ammonia, chemical supply 152 includes aqueousammonia supply 154 that provides the capability of forming aqueousammonia on demand from filtered deionized water and filtered ammonia gasin mixer 156. Water is supplied to mixer 156 through water supply line158, and ammonia gas from source 169 is supplied to mixer 156 throughammonia supply line 160. Valve 171 controls the flow of ammonia gas fromsource 169. Hot water from source 161 is provided via hot water supplyline 162, and the flow of hot water may be controlled with valve 163.Cold water from source 165 is provided via cold water supply line 164,and the flow of cold water may be controlled by valve 165. The cold andhot water can be supplied to mixer 156 separately. Alternatively,appropriate ratios of the two water flows can be combined when it isdesired to supply water to mixer 156 that has a temperature intermediatebetween the temperatures of the hot and cold water streams.

[0057] The water supplied to mixer 156 is preferably cleaned usingfilter device 166. Although any filter device could be used, the CleanPoint filtration system commercially available from Yield UpInternational is presently preferred because its combination of positiveand negative surface charged filters removes particles from the water,e.g., particles as small as 0.05 microns, with only a minimal pressuredrop. The ammonia gas from source 169 is also filtered using anappropriate gas filtration unit 168. Suitable gas filtration units arestandard in the industry and are commercially available, for example,from Millipore Corp.

[0058] Mixer 156 may be any equipment known in the art that cancontrollably combine a flow of at least one liquid with a flow of atleast one gas. Examples of equipment that could be used to accomplishsuch gas-in-liquid mixing are widely known and would include bubblers,porous septa, cascade systems, mechanical agitators, and the like. Aparticularly preferred gas-in-liquid mixer is commercially availablefrom Legacy Systems, Inc., Richardson, Tex. Other gas-in-liquid mixersare also described in U.S. Pat. No. 5,464,480, as well as in Chapter 18of Perry and Chilton, Chemical Engineer's Handbook, Fifth Edition, 1973.

[0059] The use of aqueous ammonia supply 154 as shown offers numerousadvantages. First, the use of gaseous ammonia as a solute, instead of aconcentrated ammonia solution, allows the user to prepare ultradiluteaqueous ammonia solutions with great precision over long periods oftime. Very accurate concentrations of ammonia in water can be easilyachieved under continuous, steady state conditions merely by controllingthe relative flow rates of the water and ammonia gas. For instance,ammonia concentrations as low as 17 ppm +/−0.05% can be easilyestablished and maintained. This kind of precision cannot be practicallyachieved by an approach in which water is used to dilute small volumesof more concentrated solutions. Indeed, it is the use of gaseous ammoniaas a solute that makes formation of ultradilute solutions repeatable andpractically feasible. Second, the processing liquid can be quicklyswitched from aqueous ammonia to just deionized water simply by turningoff the flow of ammonia gas. Thus, aqueous ammonia supply 154 alsoserves as a supply of deionized water. Third, the temperature of theaqueous ammonia or water, as the case may be, is easily controlledmerely by adjusting the relative flow rates of hot and cold watersupplied to mixer 156. Fourth, the ultradilute aqueous ammonia not onlycleans substrate 106, but it also cleans the surfaces of cleaning vessel102 and corresponding supply lines and inlet mechanisms that come intocontact with the ultradilute aqueous ammonia. Thus, system 100 is alsoself-cleaning in this regard.

[0060] Ozonated water is supplied from ozonated water supply 170. Theozonated water is formed by passing deionized water from valved supplyline 172 through ozonator 174 in which ozone is dissolved in the waterto form the ozonated supply. Valve 176 controls the flow of ozonatedwater to cleaning vessel 102. When an ozone generator is first started,it can take a few moments for the ozone generator to reach steady stateconditions. It is preferred, therefore, to include bypass 178 so thatthe ozonating flow can be run continuously even when there is no demandfor ozonated water in cleaning vessel 102. Flow through bypass 178 iscontrolled by valve 180.

[0061] According to a preferred mode of operation, a cascading flow ofdeionized water is established in cleaning vessel 102. The substrates106 may be positioned in cleaning vessel 102 either before or after thecascading flow is established. The water may be at any convenienttemperature ranging from 15° C. to 98° C., but most conveniently is atabout room temperature. The flow rate of rinse water can be at anysuitable rate. Generally, a flow rate of 0.1 to 100, preferably 1 to 30,more preferably about 5, gallons per minute would be suitable. Rinsingunder these conditions for 2 to 5 minutes is generally adequate.

[0062] Next, an optional cascading flow of ozonated water past thesubstrates 106 is established. Treatment with ozonated water isparticularly desirable when the surfaces being cleaned are hydrophobic,but can also be beneficial even if the surfaces are hydrophilic. Thetreatment oxidizes the surfaces, making them hydrophilic. It is believedthat cleaning performance is enhanced when the substrate surface ishydrophilic. The flow rate of ozonated water can be at any suitablerate. Generally, a flow rate of 0.1 to 100, preferably 1 to 30, morepreferably about 5, gallons per minute would be suitable. Treatmentunder these flow rates at room temperature for 2 to 5 minutes would begenerally adequate with respect to ozonated water containing 5 ppm to100 ppm, preferably 10 ppm to 50 ppm, most preferably about 17 ppm

[0063] After the optional ozone treatment, substrates 106 are againrinsed with DI water. In this case, however, the rinse water ispreferably heated to a temperature in the range from about ambient toabout 85° C. Rinsing with hot water for 2 to 5 minutes at flow rates of0.1 to 100, preferably 1 to 30, more preferably about 5, gallons perminute would be generally adequate to rinse away any remaining ozone.

[0064] After the hot water rinse is complete, a suitable flow rate ofsemiconductor grade, anhydrous ammonia gas can be turned on in order todissolve ammonia in the water and form aqueous ammonia. In preferredembodiments, the resultant concentration of dissolved ammonia (which isbelieved to actually be in the form of NH₄OH in the water) isultradilute. More preferably, suitable ultradilute aqueous ammoniasolutions are prepared by combining 0.001 ml/min to 100 ml/min, morepreferably 0.05 to 10 ml/min of ammonia (20 psi and ambient temperature)with 0.1 to 100, preferably 1 to 30, more preferably about 5, gallonsper minute deionized water. Optionally, the hot rinse water may bedumped from cleaning vessel 102 before the aqueous ammonia is introducedinto cleaning vessel 102, although cleaning performance is better if thehot rinse water is dumped first. Cleaning with the aqueous ammonia atthese flow rates and ambient temperature for 2 to 5 minutes wouldgenerally be sufficient to accomplish cleaning. After this period oftime, the substrates 106 can be pulled out of the cascading flow ofaqueous ammonia for further treatment as desired, or the aqueous ammoniacan be dumped from cleaning vessel 102 before transferring thesubstrates 106 to another treatment and/or subjecting the substrates 106to further treatment in the same cleaning vessel 102.

[0065]FIG. 2 shows system 100 which includes only a single processingvessel in which the ammonia treatment, and optionally the ozonetreatment, are carried out. However, in preferred embodiments, suchcleaning treatments are carried out in conjunction with other treatmentsthat may occur in the same vessel, in the same apparatus, but adifferent vessel, or in a different apparatus. For example, the Model6200 system commercially available from Yield Up International includesboth a megasonic cleaning vessel for carrying out ammonia cleaningtreatments as described above as well as a separate processing vesselfor carrying out STG™ rinsing and drying as described in U.S. Pat. No.5,571,337. Both the ammonia cleaning vessel and the STG™ rinse/dryvessel of the Model 6200 system are contained in the same housing toallow both the cleaning and the rinsing/drying to occur in the samecontrolled environment, thus avoiding transport from one piece ofequipment to another between treatments.

[0066] A schematic representation of the Model 6200 apparatus 200 isshown in FIG. 3. Cleaning vessel 202 and rinse/dry vessel 204 arecontained inside housing 206. Lids 208 and 210 provide access to eachvessel. Housing 206 also includes other access panels and doors (notshown) to allow componentry to be maintained, repaired, and replaced.Cleaning vessel 202 includes inner tank 212 and outer tank 214. One ormore substrates 205 are positioned inside inner tank 212 for cleaning bycascading flow(s) of one or more processing liquids. Processing liquidsare supplied to inlet 216 from chemical supply 218. Processing liquidsare introduced into inner tank 212 through sparger 220. Processingliquid is discharged from outer tank 214 through valved drain line 222.Alternatively, the contents of inner tank 212 can be quickly dumpedthrough one or more valved drain lines 223. Cleaning vessel 202 isoperationally positioned on acoustic energy source 224. A blanket 226 ofinert gas can be established over cleaning vessel 202 by introducing theinert gas through gas inlet 228. Gas can be exhausted from housing 206through valved exhaust line 230.

[0067] Rinse/dry vessel 204 includes inner tank 250 and outer tank 252.One or more substrates 254 are positioned inside inner tank 250.Processing liquids are introduced into inner tank 250 through inletmechanism 256 from chemical supply 219. Drain valve 232 can be opened toquickly dump the contents of inner tank 250 through drain line 221. Atthe bottom of inner tank 250, a baffle or mesh 234 or the like ispositioned to distribute incoming processing liquid and to help fosterlaminar flow of process liquid upward through inner tank 250. At thetop, rinse/dry vessel 204 is covered by lid 236 through which processgases may be introduced into rinse/dry vessel 204. For example, asshown, system 200 is set up so that nitrogen gas or nitrogen gascontaining trace amounts of a drying enhancement substance such asisopropyl alcohol (IPA) can be introduced into rinse/dry vessel 204. Inthis regard, nitrogen gas is supplied from nitrogen source 238. Thenitrogen can be conveyed through valved supply line 240 to container 242of liquid IPA. In container 242, the nitrogen is discharged into theliquid IPA, it bubbles upward, and then is conveyed from container 242to rinse/dry vessel 204 via valved supply line 244. As the nitrogenbubbles through the IPA, the gas picks up a trace amount of the IPA as avapor constituent. Alternatively, the nitrogen can be conveyed directlyto rinse/dry vessel 204 from nitrogen source 238 via valved supply line246. Gas supply line 249 leads to gas inlet 228. Any of supply lines240, 244, 246, and/or 249 may be heated (not shown). Also, the gasconveyed to rinse/dry vessel 204may also be filtered (not shown) ifdesired.

[0068] The present invention will now be further described with respectto the following illustrative examples.

EXAMPLE 1

[0069] Contaminated silicon wafers bearing a layer of native oxide werecleaned. Before the cleaning procedure, the wafers showed particlescounts of greater than 30,000 with respect to particles having a sizegreater than 0.16 microns. A Yield Up International Model 6200 apparatuswas used to carry out the cleaning procedure. This apparatus includes ahousing containing a cleaning vessel on the left side, and a STG™rinse/dry vessel on the right side. The cleaning vessel is a quartzcascade/overflow tank with a megasonic transducer liquid-coupled to thebottom of the tank. The STG™ rinse/dry tank is a standard Yield Up STG™rinse/dry tank of the type incorporated into the Yield Up Series 1000,2000, and 4000 systems.

[0070] A cassette of 50 wafers was placed into the empty cleaning vesselwith the wafers being substantially vertical as shown in U.S. Pat. No.5,571,337. A cleaning liquid containing approximately 100 ppm anhydrousammonia gas dissolved in filtered deionized water (i.e., the volumeratio of water to gaseous ammonia dissolved in the water was about200,000:1) was introduced into the bottom of the cleaning vessel at arate such that the surface of the cleaning liquid took 300 seconds torise from the bottom of the wafers to the top of the wafers. While theliquid level rose, megasonic energy was directed into the cleaningvessel using the system available from Kaijo Corp. The megasonic energylevel was set at “250” on the machines digital display. The water wasfiltered using the Yield Up International Clean Point® filtrationsystem. The ammonia gas was mixed into the water using the apparatusavailable from Legacy Systems, Inc. The cleaning liquid temperature wasmaintained at 60° C.

[0071] After the 300 seconds and without dumping the aqueous ammonia,the wafers were then rinsed in the same vessel with filtered deionizedwater at 60° C. for 200 seconds at a flow rate of about 5 gallons perminute. The wafer cassette was then lifted out of the rinsing water inthe cleaning vessel and transferred to the STG™ rinse/dry vessel. TheYield Up International STG™ rinse/dry procedure was then carried outusing the standard STG™ process parameters provided by the manufacturerfor use in its Model 1000, 2000, 4000, and 6000 Series systems.

[0072] The resultant dried wafers were than analyzed for post-cleanparticle counts. The wafers showed particle counts on the order of only32 particles having a size greater than 0.16 microns, demonstrating acleaning efficiency of better than 99.9%.

EXAMPLE 2

[0073] The method of Example 1 was used to treat pristine clean primewafers in order to assess the effects of the cleaning procedure upon thesurface roughness and metal contamination of the treated wafers. Surfacemicroroughness was evaluated in an area 2 micron×2 microns at the wafercenters. The results were expressed in root-mean-square (RMS), meanroughness (Ra), and peak-to-valley distance (Rmax) and are listed in thefollowing table: TABLE 1 Surface Microroughness (angstroms) before andafter cleaning. RMS Ra Rmax Pre-clean 0.8 0.6 15.8 Post-clean 0.6 0.57.3

[0074] This data shows that the cleaning operation had no adverse effectupon the surface roughness of the wafers. In fact, the waferssurprisingly were smoother after cleaning.

[0075] With regard to metal contamination, TXRF results are summarizedin the following table: TABLE 2 TXRF results (10¹⁰ atoms/cm²). Locationon wafer K Ca Ti Cr Mn Fe Ni Cu Zn Pre-clean Center <9 <6 <3 <1.5 <1.3<1.1 <0.9 <0.7 <1.4 Middle <7 <4 <5 <1.1 <1.0 <0.8 <0.7 <0.6 <0.8 Bottom<7 <6 <4 <1.5 <1.3 <1.1 <0.9 <0.8 <1.0 Post-clean Center <9 <6 <4 <1.5<1.3 <1.1 <0.9 <0.8 <1.3 Middle <7 <5 <2 <1.2 <1.0 <0.9 <0.7 <0.6 <0.8Bottom <7 <5 <5 <1.5 <1.3 <1.1 <0.9 <0.8 <1.0

[0076] These results show that no added metal contamination occurredduring the cleaning process as a practical matter.

EXAMPLE 3

[0077] For comparison purposes, the procedure of Example 1 was repeated,except no anhydrous ammonia gas was dissolved in the filtered deionizedwater. Thus, the wafers were cleaned only with deionized water. Aftercleaning, the wafers still showed a particle count of greater than30,000 with respect to particles greater in size than 0.16 microns.

EXAMPLE 4

[0078] A design experiment was performed in order to assess the impactthat acoustic energy, the cleaning enhancement substance, andtemperature have upon cleaning performance. A Yield Up InternationalModel 6200 apparatus as described in Example 1 was used to carry out theexperiment.

[0079] The experiment was conducted on respective groups of 200 mmwafers in which three wafers of each group were intentionallycontaminated with SiO₂, Si₃N₄ or W particles, respectively, using anMSP-2300 particle deposition system commercially available from MSPCorporation. Such wafer preparation has been described by Liu, et al.,Institute of Environmental Sciences Proceedings, 8-16 (1999).Approximately 20,000 to 25,000 particles in the diameter size range of0.06 micrometers to 0.3 micrometers were deposited over the entiresurface of each wafer. Particle counts before deposition (N_(i)), afterdeposition (N_(d)) and after cleaning (N_(c)) where measured utilizing aKLA-Tencor Surfscan SP1^(TBI) instrument. Particle removal efficiencieswere calculated as percent removal using the following formula:

[0080] % Removal=((Nd−Nc)/(Nd−N _(i))×100%

[0081] Four experimental runs were carried out. Run No. 1 involvedpositioning a cassette containing three contaminated wafers and onecontrol wafer into the cleaning vessel of the Yield Up InternationalModel 6200 apparatus. A cascading flow of a cleaning liquid at 60° C.and containing about 50 ppm anhydrous ammonia gas dissolved in filtereddeionized water was established. The deionized water was filtered beforeaddition of ammonia using a Yield up International Clean Point®filtration system. A heater included with the Model 6200 apparatus wasused to heat the deionized water feed to 60° C. before the ammonia gaswas added as well.

[0082] The cleaning liquid was introduced into the bottom of thecleaning vessel at a flow rate such that the surface of the cleaningliquid took about 300 seconds to rise from the bottom of the wafers tothe top of the wafers. As the liquid level rose, megasonic energy wasdirected into the cleaning vessel. The megasonic energy level was set at“450” on the machine digital display. After the 300 seconds and withoutdumping the aqueous ammonia, the of ammonia gas flow was stopped as thewafers were then rinsed in the same vessel with filtered deionized waterat 60° C. for an additional 200 seconds at a flow rate of about fivegallons per minute. The wafer cassette was then lifted out of therinsing water in the cleaning vessel and transferred to the STG™rinse/dry vessel. The Yield up International STG™ rinse/dry procedurewas then carried out using the standard process parameters provided bythe manufacturer for use in its model 1000, 2000, 4000, and 6000 seriessystems.

[0083] Run No. 2 was identical to run No. 1 except that run No. 2 wascarried out under ambient conditions without heating the deionizedwater, which was supplied at about 20° C. Run No. 3 was identical to runNo. 1 except that run No. 3 was carried out without using any ammonia inthe cleaning liquid. Run No. 4 was identical to run No. 1 except thatrun No. 4 was carried out without using any megasonic energy.

[0084] The results of this design experiment are shown in the followingtables: TABLE 1 Particle Removal Efficiencies SiNO₂ Particles ParticleSize 60° C. 20° C. No NH₃ No Meg >0.06 μm 82% 86% 76% 16% >0.12 μm 73%81% 51% 15%

[0085] TABLE 2 Particle Removal Efficiencies Si₃N₄ Particles ParticleSize 60° C. 20° C. No NH₃ No Meg >0.06 μm 81% 84% 61% 32% >0.12 μm 65%67% 25% 13%

[0086] TABLE 3 Particle Removal Efficiencies W Particles Particle Size60° C. 20° C. No NH₃ No Meg >0.06 μm 91% 96% 68% 84% >0.12 μm 79% 94%50% 82%

[0087] Surprisingly, run No. 2, which was carried out at 20° C., yieldedhigher particle removal efficiencies that run No. 1, which was carriedout at 60° C. This result was unexpected, because most researchers havereported that particle removal efficiencies increased with an increasein process temperature when utilizing SC-1 cleaning chemistry. Thepositive relationship between expected removal efficiency andtemperature is based upon the conventional belief that particle removalis accomplished via an etching mechanism when SC-1 chemistry is used forcleaning. According to conventional wisdom, therefore, reduced particleremoval efficiencies would have been expected at 20° C. as compared to60° C. because etching rate decreases with decreasing temperature. Thefact that the ultradilute ammonia/megasonic process of the presentinvention was better when carried out at 20° C. indicates that thecleaning mechanism of the present convention is not an etching mechanismsuch as is associated with SC-1 chemistry. While note wishing to bebound by theory, it is believed that electrostatic repulsion forces, notetching, plays a significant role in the ultradilute chemistry of thepresent invention.

[0088] Without ammonia, run No. 3 demonstrates a significant decrease inparticle removal efficiencies. The results suggest that the megasonicenergy, in the absence of ammonia, might help to detach a large numberof surface-bound particles from a wafer. But then a significant numberof the dislodge particles are able to be redeposited back onto the wafersurface. While not wishing to be gone by theory, it is believed that theparticles and the wafer surfaces tend to be oppositely charged. As aconsequence, electrostatic attraction, at least in part, is causing theparticles to settle back onto the wafer surfaces.

[0089] Without acoustic energy such as megasonic energy, run No. 4 alsodemonstrates a significant decrease in particle removal efficienciesThese results suggest that the megasonic energy help to break thesurface adhesion forces between the particles and the wafer surfaces.The results also suggest that such forces are greater for SiO₂ and Si₃N₄than for W.

[0090] In summary, the results from this experiment indicate that theultra-dilute ammonia/megasonic process for the present convention reliesupon the megasonic energy to help detach surface bound particles whilethe high pH of the ultra-dilute, aqueous ammonium hydroxide cleaningliquid helps to prevent reattachment of the particles to the wafersurface. Further, the process appears to be more effective at lowertemperature, e.g., 20° C. as compared to higher temperature, e.g., 60°C.

What is claimed is:
 1. A method of cleaning a surface of an article,comprising the steps of: dissolving an ultradilute concentration of agaseous cleaning enhancement substance in a liquid solvent to form acleaning liquid; causing the cleaning liquid to contact the substratesurface; and while causing the cleaning liquid to contact the substratesurface, applying acoustic energy to the cleaning liquid.
 2. The methodof claim 1, wherein the volume ratio of the solvent to the cleaningenhancement substance is in the range from about 500:1 to about500,000:1.
 3. The method of claim 1, wherein the volume ratio of thesolvent to the cleaning enhancement substance is in the range from about1000:1 to about 300,000:1.
 4. The method of claim 1, wherein the volumeratio of the solvent to the cleaning enhancement substance is in therange from about 100,000:1 to about 200,000:1.
 5. The method of claim 1,wherein the cleaning liquid is caused to contact the surface of thearticle under laminar flow conditions.
 6. The method of claim 1, whereinthe cleaning liquid is prepared by a method comprising dissolving anultradilute concentration of gaseous anhydrous ammonia into deionizedwater.
 7. The method of claim 1, wherein the step of contacting thearticle surface with the cleaning liquid comprises causing a risingsurface of the cleaning liquid to move upward and across the substratesurface.
 8. The method of claim 1, wherein the article surface comprisessilicon, silicon oxide, or combinations thereof.
 9. The method of claim1, wherein the acoustic energy comprises megasonic energy.
 10. Themethod of claim 1, further comprising the steps of: causing a cascadingflow of a rinse liquid to contact the cleaned article surface; causingat least one process gas to contact the rinsed surface.
 11. The methodof claim 10, wherein the step of causing at least one gas to contact therinsed surface comprises the steps of: causing a processing reagentcomprising a gas carrier and a cleaning enhancement substance to contactthe article surface; and causing a drying reagent to contact the articlesurface, wherein the drying reagent comprises a heated gas.
 12. Themethod of claim 11, wherein the gas carrier comprises nitrogen and thecleaning enhancement substance comprises isopropyl alcohol.
 13. Themethod of claim 12, wherein the concentration of isopropyl alcohol inthe carrier gas is ultradilute.
 14. The method of claim 1, wherein thecleaning liquid is at about ambient temperature during at least aportion of the time that the cleaning liquid is caused to contact thesubstrate surface.
 15. The method of claim 6, wherein the cleaningliquid is at about ambient temperature during at least a portion of thetime that the cleaning liquid is caused to contact the substratesurface.
 16. The method of claim 1, wherein the cleaning liquid is atabout 60° C. during at least a portion of the time that the cleaningliquid is caused to contact the substrate surface.
 17. The method ofclaim 6, wherein the cleaning liquid is at about 60° C. during at leasta portion of the time that the cleaning liquid is caused to contact thesubstrate surface.
 18. A method of cleaning a surface of an article,comprising the steps of: dissolving an ultradilute concentration ofgaseous ammonia into an aqueous solvent to form a cleaning liquidcomprising aqueous ammonia; and causing the aqueous ammonia cleaningliquid to contact the surface of the substrate.
 19. The method of claim18, wherein the cleaning liquid is at about ambient temperature duringat least a portion of the time that the cleaning liquid is caused tocontact the substrate surface.
 20. The method of claim 18, wherein thecleaning liquid is at about 60° C. during at least a portion of the timethat the cleaning liquid is caused to contact the substrate surface. 21.The method of claim 18, wherein the volume ratio of the solvent to thedissolved ammonia is in the range from about 500:1 to about 500,000:1.22. The method of claim 18, wherein the volume ratio of the solvent tothe dissolved ammonia is in the range from about 1000:1 to about300,000:1.
 23. The method of claim 18, wherein the volume ratio of thesolvent to the dissolved ammonia is in the range from about 100,000:1 toabout 200,000:1.
 24. The method of claim 18, wherein the step ofcontacting the article surface with the cleaning liquid comprisescausing a rising surface of the cleaning liquid to move upward andacross the substrate surface.
 25. A method of cleaning a surface of anarticle, comprising the steps of: contacting the surface of thesubstrate with a first processing liquid comprising an oxidizing agent;and after contacting the surface with the first liquid, contacting thesurface of the substrate with a second liquid comprising a dissolvedconcentration of a gaseous cleaning enhancement substance, said contactwith the second liquid occurring at least partially in the presence ofacoustic energy.
 26. The method of claim 25, wherein the cleaning liquidis at about ambient temperature during at least a portion of the timethat the cleaning liquid is caused to contact the substrate surface. 27.The method of claim 25, wherein the cleaning liquid is at about 60° C.during at least a portion of the time that the cleaning liquid is causedto contact the substrate surface.
 28. The method of claim 25, whereinthe volume ratio of the solvent to the cleaning enhancement substance isin the range from about 500:1 to about 500,000:1.
 29. The method ofclaim 25, wherein the volume ratio of the solvent to the cleaningenhancement substance is in the range from about 1000:1 to about300,000:1.
 30. The method of claim 25, wherein the volume ratio of thesolvent to the cleaning enhancement substance is in the range from about100,000:1 to about 200,000:1.
 31. The method of claim 25, wherein thefirst processing liquid is ozonated.
 32. The method of claim 25, whereinthe cleaning liquid is obtained by a method comprising dissolving anultradilute concentration of gaseous anhydrous ammonia into deionizedwater.
 33. The method of claim 25, wherein the step of contacting thearticle surface with the cleaning liquid comprises causing a risingsurface of the cleaning liquid to move across the substrate surface. 34.A method of cleaning a surface of an article, comprising the steps of:contacting the surface with ozonated water; after contacting the surfacewith ozonated water, contacting the surface with an aqueous liquidcomprising an ultradilute concentration of ammonia, said contact withthe aqueous liquid occurring at least partially in the presence ofacoustic energy.
 35. The method of claim 34, wherein the cleaning liquidis at about ambient temperature during at least a portion of the timethat the cleaning liquid is caused to contact the substrate surface. 36.The method of claim 34, wherein the cleaning liquid is at about 60° C.during at least a portion of the time that the cleaning liquid is causedto contact the substrate surface.
 37. The method of claim 34, whereinthe volume ratio of the aqueous liquid to the ammonia is in the rangefrom about 500:1 to about 500,000:1.
 38. The method of claim 34, whereinthe volume ratio of the aqueous liquid to the ammonia is in the rangefrom about 1000:1 to about 300,000:1.
 39. The method of claim 34,wherein the volume ratio of the aqueous liquid to the ammonia is in therange from about 100,000:1 to about 200,000:1.
 40. The method of claim34, wherein the step of contacting the article surface with the cleaningliquid comprises causing a rising surface of the cleaning liquid to moveupward and across the substrate surface.
 41. A method of cleaning asurface of a microelectronic substrate, comprising the steps of:positioning the substrate in a vessel such that the surface of thesubstrate is substantially vertical; introducing a cleaning liquid intothe vessel such that a top surface of the cleaning liquid rises whilethe top surface of the cleaning liquid traverses the substrate surface,wherein the cleaning liquid comprises an ultradilute concentration of acleaning enhancement substance; and applying acoustic energy to therising cleaning liquid.
 42. A method of cleaning a surface of anarticle, comprising the steps of: dissolving an ultradiluteconcentration of a gaseous cleaning enhancement substance in a liquidsolvent to form a cleaning liquid; causing the cleaning liquid tocontact the substrate surface; and while causing the cleaning liquid tocontact the substrate surface, applying acoustic energy to the cleaningliquid; after causing the cleaning liquid to contact the substratesurface, rinsing the substrate surface; after rinsing the substratesurface, drying the substrate surface.
 43. The method of claim 42,wherein the cleaning liquid is at about ambient temperature during atleast a portion of the time that the cleaning liquid is caused tocontact the substrate surface.
 44. The method of claim 42, wherein thecleaning liquid is at about 60° C. during at least a portion of the timethat the cleaning liquid is caused to contact the substrate surface. 45.The method of claim 42, wherein the volume ratio of the solvent to thecleaning enhancement substance is in the range from about 500:1 to about500,000:1.
 46. The method of claim 42, wherein the volume ratio of thesolvent to the cleaning enhancement substance is in the range from about1000:1 to about 300,000:1.
 47. The method of claim 42, wherein thevolume ratio of the solvent to the cleaning enhancement substance is inthe range from about 100,000:1 to about 200,000:1.
 48. The method ofclaim 42, wherein the cleaning liquid is prepared by a method comprisingdissolving an ultradilute concentration of gaseous anhydrous ammoniainto deionized water.
 49. The method of claim 42, wherein the step ofcontacting the article surface with the cleaning liquid comprisescausing a rising surface of the cleaning liquid to move upward andacross the substrate surface.
 50. The method of claim 42, wherein theacoustic energy comprises megasonic energy.
 51. The method of claim 42,wherein the drying step comprises: causing a processing reagentcomprising a carrier gas and a cleaning enhancement substance to contactthe article surface; and causing a drying reagent to contact the articlesurface, wherein the drying reagent comprises a heated gas.
 52. Themethod of claim 51, wherein the gas carrier comprises nitrogen and thecleaning enhancement substance comprises isopropyl alcohol.
 53. Themethod of claim 52, wherein the concentration of isopropyl alcohol inthe carrier gas is ultradilute.
 54. A method of cleaning a surface of anarticle, comprising the steps of: dissolving an ultradiluteconcentration of a gaseous cleaning enhancement substance in a liquidsolvent to form a cleaning liquid; causing a cleaning liquid to contactthe substrate surface, wherein the cleaning liquid comprises anultradilute concentration of a cleaning enhancement substance, saidcleaning liquid during at least a portion of said contact being at atemperature below about 30° C.; and while causing the cleaning liquid tocontact the substrate surface, applying acoustic energy to the cleaningliquid.
 55. The method of claim 54, wherein the temperature of thecleaning liquid is in the range from about 0° C. to about 25° C.
 56. Themethod of claim 54, further comprising the step of dissolving anultradilute concentration of a gaseous cleaning enhancement substance indeionized water to form the cleaning liquid.
 57. The method of claim 56,wherein the gaseous cleaning enhancement substance comprises ammoniagas.
 58. The method of claim 54, wherein the cleaning enhancementsubstance is aqueous ammonium hydroxide.